1. Field of the Invention
The present invention relates to a zinc- or zinc alloy-coated steel sheet having an excellent spot-weldability, and to a process for producing same.
2. Description of the Related Art
Processes for improving the spot-weldability of zinc- or zinc alloy-coated steel sheets are disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 55-110783 and 60-63394, for example, in which an oxide film such as Al.sub.2 O.sub.3 is formed on a zinc- or zinc alloy-coated steel sheet and a high electric resistivity of the oxide film is utilized to promote welding and prevent contact between an electrode tip and the steel sheet to be welded, with the result that the tip loss by fusion is prevented to increase the tip life.
Another method for improving the spot-weldability is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-104463, in which a coated steel sheet is heat-treated to form thereon an oxidized film having a ZnO/Zn ratio of from 0.1 to 0.70.
These publications, however, do not give satisfactory results in the practical manufacturing process, and a further improvement is required for the spot-weldability of the coated steel sheet.
Further, a conventional process effective for any kind of zinc- or zinc alloy-coated steel sheet and a process available for various kinds of such steel sheets has not been developed, despite the need for same.